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Short-time dynamics of superfluids far from equilibrium remains largely unknown, despite its importance for key processes in these systems. Here, we describe a method for locally perturbing the density of superfluid helium via the excitation of roton pairs with ultrashort laser pulses. By measuring the time dependence of this perturbation, we track the nonequilibrium dynamics of the two-roton states on femtosecondandpicosecond timescales. Our results reveal an ultrafast equilibration of roton pairs as they thermalize with the colder equilibrium quasiparticle gas. Future applications of this technique to different temperature and pressure regimes, in various superfluids, will enable to probe rapid nucleation and decay processes, as well as metastable Bose-Einsteincondensatesof rotons and roton pairs. [ABSTRACT FROM AUTHOR]

While roton dispersion relations had been restricted to correlated quantum systems at low temperature, recent works show the possibility of obtaining this unusual dispersion in acoustic and elastic metamaterials. Such phenomenon has been demonstrated in periodic structures by means of beyond-nearest-neighbour interactions, following the formulation firstly developed by Brillouin in the ′ 50 s. In this paper, we demonstrate both numerically and experimentally that beyond-nearest-neighbour connections are not a necessary condition to obtain this unusual dispersion relation in elasticity. Leveraging the intrinsic complexity of elastic systems supporting different types of waves, we demonstrate that mode locking can be applied to obtain roton dispersion, without the need of elastic or magnetic interactions between non nearest neighbours. Moreover, the combination of roton dispersion and rainbow physics enables spatial separation of the energy fluxes with positive and negative group velocity. • Development of a periodic structure with roton-like dispersion without implementing long range interactions. • Roton-like dispersion using locking coupling mechanisms between counter propagating waves in periodic arrays. • Graded roton-like structure for achieving a double Rainbow trapping effect. • Numerical and experimental validation of all the above statements. [ABSTRACT FROM AUTHOR]

We present a microscopic many-body argument for supersolidity. We suggest that super-solidity can be realized in 20Ne. [ABSTRACT FROM AUTHOR]

Abstract: Two mechanisms of transition of the superfluid liquid 4He to quantum turbulence regimes are proposed for the case when the influence of the normal fluid on superfluid flow is suppressed by introducing superleaks at the ends of the capillary. Using dimensional analysis it is found that in the roton mechanism the critical velocity depends on channel size as , matching the experiments. For the second, super-flow mechanism, the analysis of independent parameters relevant for this phenomena leads to critical velocity depending on d as . It is shown that turbulence for super-flow mechanism arises when a “quantum Reynolds number” exceeds some critical value which is about 103 for 1D geometry. The dimensional analysis of the equation for critical velocity of superfluid flow without superleaks at the ends of the capillary is also presented. [Copyright &y& Elsevier]

There has been a resurgence of interest in the properties of solid helium due to the recent discovery of non-classical rotational inertia (NCRI) in solid 4He by Chan and coworkers below 200 mK which they have interpreted as a transition to a `supersolid' phase. We have carried out a series of elastic and inelastic neutron scattering measurements on single crystals of hcp 4He at temperatures down to 60 mK. While we have found no direct evidence of any change in the excitation spectrum at low temperatures, we have found that the excitation spectrum of solid 4He shows several interesting features, including extra branches in addition to the phonon branches. We interpret these extra branches as single particle excitations due to propagating vacancy waves, which map on to the famous `roton minimum' long known in the excitation spectrum of superfluid liquid 4He. The results show that in fact solid 4He shares several features in common with the superfluid. [ABSTRACT FROM AUTHOR]